Biosensors are measurement devices which utilize an excellent molecular recognizing ability of organisms or biomolecules. Combinations of biological substances which have affinity each other include, for example, enzyme-substrate, antigen-antibody, DNA-DNA etc. Biosensors utilize the principle where one substance of the above respective combination which is immobilized or supported on a substrate is used, thereby making it possible to selectively measure the other substance. In recent years, biosensors are expected to have wide application not only in medical fields but also in environment, foods etc. In order to increase the applicable area thereof, there is a need for small, light and highly sensitive biosensors which can be placed in any location or is portable.
Currently, as one of the highly sensitive sensing methods, plasmon sensors which utilize an interaction between existing plasmons of a metal surface or metal microparticles and light have been vigorously studied.
Sensors using conventional surface plasmon resonance (SPR (Surface Plasmon Resonance) sensors) utilize a phenomenon where the only light entered from a specific angle resonance with metal surface plasmons and is absorbed when the light is entered into a metal thin layer surface. The angle at which this absorbance occurs is sensitive to the surface condition of a metal thin layer (refraction index), and a reaction which occurs at a metal surface (e.g. antigen-antibody reaction) etc. can be measured by measuring an intensity of reflected light while changing an angle of incidence.
However, these SPR sensors require a prism in the construction, hence are complex in optics system. Therefore, it is considered that there is a limitation for miniaturization.
Japanese Patent Application Laid-Open No. 2000-035685 discloses a sensor which utilizes localized plasmon resonance by metal microparticles. The sensor disclosed in Japanese Patent Application Laid-Open No. 2000-035685 is a localized surface plasmon resonance (LSPR) sensor which detects a refraction index of a medium in the vicinity of metal microparticles by measuring an absorbance of light which passes through metal microparticles immobilized as a film on a substrate surface. It is considered that this sensor unit does not require a prism, can be placed at narrow spaces and can be applied on a substrate having a curved surface.
In J. Phys. Chem. B, vol. 103, p. 3073 (1999), nanorods are prepared by using gold. It discloses spectra based on localized plasmon resonance for these nanorods, and the relationship between an aspect ratio and a spectrum of nanorods is discussed therein.
In Japanese Patent Application Laid-Open No. 2000-035685, gold microparticles are immobilized on a surface-treated substrate by soaking the substrate in a solution of gold collides having a diameter of about 20 nm. The shape of gold microparticles may be close to the spherical shape. The fact is that enough sensitivity is not necessarily obtained with a plasmon sensor merely using these gold microparticles.
J. Phys. Chem. B, vol. 103, p. 3073 (1999) also discloses that the maximum absorbance wavelength is changed by making gold into a rod shape (columnar shape) and varying an aspect ratio of gold rods. However, J. Phys. Chem. B, vol. 103, p. 3073 (1999) does not disclose about orientation of gold rods and further improvement in the sensitivity is desired.